iStockphoto/Thinkstock(ASPEN, Colo.) -- Al Qaeda's most diabolical bomb maker, who has targeted the American homeland at least four times, has trained other terrorists who are now being hunted down, the top U.S. aviation security official said on Friday.

Transportation Security Administrator John Pistole told ABC News during a discussion at a counterterrorism conference that accused Saudi terrorist Ibrahim al-Asiri had shared his expertise at building almost undetectable bombs with a number of al Qaeda operatives.

"There is intel that he has unfortunately trained others," Pistole said at the annual Aspen Security Forum.

Asiri, 31, created two versions of an improvised explosive device hidden in men's underwear, with which the Yemen-based al Qaeda affiliate al Qaeda in the Arabian Peninsula (AQAP) almost succeeded in blowing up a passenger jet over Detroit on Christmas Day 2009. A "new and improved" version was obtained by spy agencies last year.

Pistole said that "there is a lot of effort to identify" those to whom Asiri has taught his terror tradecraft, particularly after similar liquid explosives bombs were discovered aboard U.S.-bound cargo jets in 2010.

However, "talent is not always transferable," a U.S. counterterrorism official told ABC News recently, meaning that Asiri's skills as a bomb innovator are considered unique.

Pistole also for the first time detailed the sophisticated "Underwear Bomb II," which a double-agent stole from AQAP in 2012 after the group dispatched him as a suicide bomber aboard an aircraft. ABC News obtained a Department of Homeland Security illustration of the newer bomb.

Pistole called the U.S. ally-led operation an "intelligence coup," because it enabled western counterterrorism services to thwart a serious threat from a bomb almost impossible to detect by magnetometers and advanced imaging machines at airports.

"It was a new type of explosive we had never seen," which used two redundant initiators filled with liquid explosives to detonate a larger liquid explosive charge in men's briefs, Pistole said.

"He encased this explosive in caulk -- kitchen caulk," he explained, to conceal the scent of he explosives and "evade bomb sniffing dogs."

It "possibly" could have been detected by the TSA, but that would be less likely in overseas airports, where security screening isn't always as precise as it is inside the U.S., Pistole added.